The present disclosure relates to a method for complete machining of at least unmachined journals and flat shoulders of forged or cast blanks of a crankshaft and to a production line for carrying out this method.
Since crankshafts constitute a central component in particular in reciprocating internal combustion engines and this type of internal combustion engine has been used successfully for decades, for an equally long time the manufacturing technology has been concerned with an improvement not only in the accuracy of production but also the economics of production.
One difficulty in the complete machining of crankshafts is that the crankshaft blanks which are frequently supplied as forgings or castings must be subjected to machining not only of the main bearing journals and crankpin journals, but also the flat faces of the flanks, the flat end faces and also a connecting flange and centring and/or oil supply holes.
The forged or cast blanks have a relatively large oversize, in particular at the crucial locations such as for example main bearings, pin bearings, flat shoulders, flange bearings, which must be removed by machining. In order that a blank can be subjected to subsequent machining, this blank must be machined with flat faces flat at its ends, and centring holes must be introduced there. This is necessary in order that the crankshaft to be machined can be gripped exactly and reproducibly on the respective machine tools in order to achieve a high manufacturing accuracy.
In the context of this present disclosure blanks are forged or cast blanks which are merely machined with flat faces on their ends and have centring holes.
In the case of blanks for a crankshaft, the main bearings, pin bearings, grooves and recesses, flange bearings, flange and pin diameter, optionally concentric profiles and side flank surfaces and outer flank diameter including chamfers, must be machined. In addition, crankshafts must be provided with oil holes so that numerous manufacturing steps are necessary in order to make a finished completely machined component from a forged or cast crankshaft blank.
The course of technical development with respect to the manufacture of crankshafts has concentrated on largely combining the different technological manufacturing operations, such as milling, turning, grinding and drilling, as far as possible on individual manufacturing centres. This development also included the trend towards increasing the proportion of grinding operations in the number of operations for complete production of the crankshaft, above all things because of the high manufacturing accuracy which can be achieved thereby. Nevertheless turning and grinding machines are combined in production lines for complete machining of crankshafts, in order in particular that those operations which can be produced more favorably by turning are carried out on a turning machine, but those operations which can be produced more precisely and better on grinding machines are carried out on these grinding machines.
Because of the relatively large oversize, which may amount to 5 mm and more, the machining of a crankshaft differs according to the design, size, hardness, type and batch size. In any case the machining begins with rough machining, in which for example the crankshaft is milled at the ends to the correct length and centring holes are also introduced at these ends. This is followed by machining of the main bearing surfaces and also of the pin bearings. Turning and milling, in particular trochoidal milling, as well as turn broaching are widely used as machining techniques for this rough machining. For the main bearing surfaces, for example, rough grinding may also be carried out. Pin bearings are being predominantly milled. Turn broaching and also grinding are likewise used.
Complete machining of a crankshaft includes numerous further method steps such as drilling of oil channels, which can be performed for example using deep drilling techniques, induction hardening for improvement of the wear resistance, as the bearing running surfaces are hardened and the fatigue strength is increased by generation of compressive residual stresses in particular in the radii. This operation is performed by heating, quenching and tempering. Further technological processes are rolling of the radii or radius induction hardening in order to increase the fatigue strength. Likewise nitriding of a finish-machined crankshaft can be used for improvement of the wear and fatigue behavior. Main bearings and pin bearings are frequently also ground, as are the ends of the crankshafts, i.e. the journals and flange ends thereof. Grinding has particular advantages in terms of roundness, surface quality, straightness, optionally iconicity, dimensional stability, etc.
In the context of the present disclosure, complete machining should be understood to mean complete machining of unmachined central journals as well as unmachined crankpin journals and also the respective flat shoulders, which are likewise unmachined and surround the journals, of forged or cast blanks of a crankshaft. Although the present disclosure does include all the operations for complete machining of a crankshaft in the broadest sense, the crux of the present disclosure extends to the above-mentioned machining of journals and flat shoulders on forged or cast blanks of a crankshaft.
In the prior art it is known in principle to machine flat faces. In known installations these machining operations are preferably carried out by means of trochoidal milling or turn broaching. In this case hard metal or polycrystalline diamond plates are disposed on a disc. With such machines it is possible, when machines with one or two tool headstocks are used, for one or two bearing points in each case to be machined one after the other and sometimes simultaneously. Such machines are not only expensive to procure, machining of individual bearings also leads to relatively long machining times, so that for known crankshaft production lines a considerable number of these machines is necessary, which makes the crankshaft manufacture very expensive overall.
The trochoidal milling or turn broaching has found widespread use in the manufacture of crankshafts. The disadvantages of these methods, i.e. trochoidal milling or turn broaching, often reside in the fact that that in principle these methods are carried out dry, i.e. a cooling liquid is not used. Furthermore the tools for the turn broaching and trochoidal milling are very complicated, because these are disc-like components which have plates which produce the actual free-cutting function on the end faces in the circumferential region. These plates must be individually adjusted very precisely so that during the free-cutting machining the most uniform material removal possible is carried out, and on the other hand the plates are also uniformly loaded. Since these plates are disposed at defined intervals on the periphery, a relatively untidy surface contour is also produced which may also be designated as scaly. Under these circumstances the result of the rough machining has a negative effect on the subsequent fine machining operations, because the tools used therefore initially have to offset the rough and uneven surface. This in turn presupposes that a relatively large oversize must be maintained for the final machining as a result of the rough machining. The tools used for trochoidal milling or turn broaching also require long periods of time in order to replace the worn plates. Several hours are frequently necessary for this. The makes the production process more expensive. However, since crankshafts are frequently mass-produced cost-effective solutions must be sought.
During the rough machining it is known that a relatively large amount of material is removed, which, in particular in the case of methods with dry machining, leads to a substantial heat input into the crankshaft to be machined. This substantial heat input leads, in addition to the high machining forces because of the engagement of the tools on the component to be machined, to a deformation of the component with a disadvantageous effect on the accuracy to be achieved later. The heat input into the crankshaft as a result of the rough machining also leads to the relief of stresses in the component, which likewise lead to distortions of the component. Furthermore the rough machining leads to significantly higher loads on the tools in this technological step by comparison with fine or final machining.
Therefore attempts have been made to replace the trochoidal milling and turn broaching by grinding operations. However, when machining such blanks it should be noted that, because of the necessary large amount of material to be removed, pre-grinding operations are generally associated with relatively high wear on the grinding wheels (in particular on the flat faces).
The grinding itself will generally be carried out as a wet machining operation, because this is performed with cooling lubricant. Thus less heat is introduced into the component by the grinding, and also the machining forces are lower, so that the deformations due to the processing on the crankshaft during grinding can be reduced. The has a direct positive effect on the accuracy of the crankshaft, so that truer running of a finished crankshaft can be achieved.
This is particularly critical when machining the flat shoulders of the central journal regions, because these flat shoulders have a significantly greater height than the flat shoulders on the crankpin journal of the crankshaft. For the trochoidal milling method or the turn broaching method this means that the numerous cutting plates disposed on the external periphery of the tool undergo substantial unilateral loading during the machining of the flat shoulders, whereas large parts of these cutting plates do not come into engagement at all with the workpiece to be ground, i.e. the crankshaft, during the machining of the flat shoulders. As a result the machining tools wear relatively quickly and have to be fitted with new cutting plates at comparatively short intervals. The leads to an increase in the cost of the production process because the fitting of new parts is a time-consuming operation. Only after the flat shoulders have been completely machined do the other regions of the trochoidal milling or turn broaching tools take part in the machining as they machine the immediate journal regions. As already described above, the result of the machining by turn broaching or trochoidal milling is a relatively “scaly” surface. Such an irregular and relatively rough surface has considerable disadvantages for subsequent finish machining operations. If the final machining operations are performed as usual by grinding, then the tools employed there must engage in a relatively uneven and rough surface, which leads to considerable wear on the grinding wheels used therefor. Because of the only relatively harsh surface which can be achieved by this pre-machining by trochoidal milling or by turn broaching, a relatively large oversize with respect to the final dimension to be achieved must be maintained after the rough machining so that all errors can still be compensated for by means of final machining. The crankshaft blanks machined in this way are also designated as pre-machined crankshafts. For these pre-machined crankshafts a conventional finish machining is then carried out in such a way that first of all pre-grinding is carried out, followed by finish grinding or fine grinding. These two methods are generally carried out by two different grinding wheels, because on the one hand a relatively large oversize is still to be abraded, which for a fine grinding machine would constitute an excessive strain, and because on the other hand the surface is relatively rough, which would likewise lead to relatively fast wear of a fine-grinding wheel, with the negative result that the high quality requirements for a crankshaft could not be attained or could be attained only with difficulty.
Attempts have been made to minimize this disadvantage, by the use of mixed methods for machining crankshaft blanks, i.e. by combining methods consisting turn broaching and trochoidal milling as well as grinding with one another in such a way that in the case of a main bearing the flat faces have been machined by means trochoidal milling or turn broaching, followed by grinding of the main bearing journal, so that a steady rest for the further machining operations can be used there. Ultimately, these combined methods still have the disadvantages which are manifested directly on the component in connection with the trochoidal milling and turn broaching. These disadvantages are the high loads also resulting from dry machining, high surface roughness and the deformations of the crankshaft as a result of this machining.
In order to eliminate the above-mentioned disadvantages of the previously customary machining methods for crankshafts in the rough machining such as turn broaching and trochoidal milling, attempts have been made to completely grind the crankshafts, i.e. to perform the rough machining by rough grinding.
In the known production methods for crankshafts, in which the crankshaft blanks are already roughly pre-machined, i.e. in which both flat surfaces and also journal regions for example have also been roughly pre-ground, the main focus was the finish machining of the crankpin journal and also the main bearing journal as well as the flat shoulders on a finish grinding machine which performs the operations of pre-grinding and finish grinding. Already the sequence of the machining of crankpin journals and main bearing journals has a effect on the precision to be achieved later. Thus this section of the method has not only attracted the highest attention, but also special grinding wheels have been developed for carrying out these complex grinding operations, including such grinding wheels, in which a readjustment of the width of the grinding wheels was possible. This shows that, in order to ensure above all the quality of the crankshafts, in particular the grinding operations have been perfected.
In spite of this perfected mode of production in the field of grinding it has been ascertained that in particular the wear on grinding wheels as a result of the grinding of flat shoulders and bearing journals is unsatisfactorily high. This applies in particular to the large flat shoulders on the central journal regions and in particular to the rough grinding operation. This rough grinding was performed by plunge grinding, so that both flat shoulders which enclose the respective journal between them are ground simultaneously. If the grinding wheel is plunged into this region for purpose of grinding the flat shoulders, basically only the direct edge regions of the grinding wheel are most highly loaded, whereas the abrasive grains disposed there between over the width of the grinding wheel do not have to perform any grinding work. This leads to substantial wear on the rough-grinding wheels, so that the service life of this grinding wheels is unacceptably short. Therefore in this type of rough grinding only as much as is absolutely necessary of the oversize on the central bearing points is removed by grinding, so that a considerable oversize still remains for the final machining. Improved grinding wheel materials have not provided a sufficient remedy for this problem.
The considerable disadvantages with regard to precision, required performance and cycle time as well as costs of the methods of turn broaching and trochoidal milling, rough grinding and a combination thereof do not therefore meet the existing requirements in particular for mass production.
Therefore the object of the present disclosure is to provide or optimize a method for complete machining of crankshaft blanks which are, if need be, pre-machined with flat faces at the ends, as well as a production line necessary for carrying out the method with the appropriate production operations for mass production in such a way that the tool wear as well as the tool costs and thus the unit costs of the crankshafts can be reduced and the total production process can take place more cost-effectively, and with high production precision of the crankshaft.
Surprisingly it has been shown according to the present disclosure that—contrary to the previous course of the technical development—by separation or sub-division of the different steps of the total production process, namely by turning away from the hitherto conventional procedure, the aforesaid problems can be remedied, in that above all the rough grinding, turn broaching or trochoidal milling part of the rough machining which includes the grinding of the large flat shoulders on the central journals, is instead subjected to a normal turning operation. Since the rough-grinding wheels and the tools for turn broaching or trochoidal milling now no longer have to be used for machining the flat shoulders on the central journal regions, the service life thereof is extended considerably. However, the tools are also subjected to a more uniform loading, since in the rough machining of only the journals the cutting plates or the grinding wheel grains come into engagement immediately and totally. A special advantage is offered by the rough grinding in particular of the central journals, after the flat shoulders thereof have been turned. During the rough grinding may the cutting speed can be significantly increased relative to the rotary milling or turn broaching, a fixed tool being present in the case of turn broaching. In spite of the increased cutting speed the heat input during grinding is considerable lower, so that the crankshaft undergoes less deformation and so that in addition to a smoother and more uniform surface a lesser run out can be achieved. Thus in the case of rough grinding according to the present disclosure of only the central journal regions on the blank a greater part of the oversize present on the unmachined blank can be ground by means of the grinding with cooling and grinding oil being supplied. This means that with the method according to the present disclosure the rough machining can be carried out to far into the part of the oversize which hitherto had to be provided for the known finish grinding. Thus the rough grinding not only relieves the load on the tools during final grinding, but therefore also during finish grinding considerably less material has to be removed in order to grind to the final dimension. Therefore during finish grinding this can take place in a single operation, so that the previously conventional sub-division into pre-grinding with a pre-grinding wheel and finish grinding with a finish-grinding wheel is often no longer necessary. Instead grinding can be performed in a single operation with one single fine-machining grinding wheel without this latter being overloaded. Because in the fine grinding less of the oversize is to be removed, the loads on the crankshaft during the finish grinding are reduced, which has a directly positive effect on the achievable quality of the crankshaft.
The combination according to the present disclosure of turning the flat shoulders and rough grinding of the journal regions is carried out by an intelligent sub-division of these production processes in the context of optimizing the deformation of the crankshaft during the machining and in the context of optimizing the oversize. With regard to the technological operating sequence, according to the present disclosure those parts (the flat shoulders of the central journals) which can be produced significantly more effectively by turning in an optimal manner of workpiece deformation, heat input, machining precision are turned, whereas the rest of the machining is relieved of high loads. Thus with regard to the technological operating sequence turning is deliberately employed again and, as it were, freed from a complete grinding machining in order to be able to carry out the total production process overall more cost-effectively and with a higher quality of the workpiece. The grinding is carried out by means of rough grinding machining more precisely overall than machining by means of grinding of flat shoulders and journal regions in a single operation by means of a plunge grinding operation.
In the case of pin bearings, on the other hand, rough grinding is retained for both the flat shoulders there and also the crankpin journals. On the one hand a machine for turning a section which moves eccentrically when the crankshaft rotates would be difficult to design, whilst on the other hand the flat shoulders on the crankpin are clearly smaller than those on the central journals. In addition, in the case of the pin bearings a somewhat greater oversize must be retained from the outset after the rough machining because of the angle error which occurs. It must still be possible to compensate for this angle error in the final machining. Therefore it is also tolerable to grind the flat shoulders at the same time, since the negative effect of the wear on the grinding wheel is clearly less in the case of the pin bearing. This is taken into account in the context of the overall process optimization.
Accordingly, rough grinding within the context of the present disclosure is the machining of the central journals, optimized in relation to the oversize, and thus the relief of the load on the finish grinding operation to be carried out thereafter. For the forged or cast blanks oversizes of in some cases more than 5 mm are quite usual. In known methods in which prefabricated crankshafts are machined, the pre-machining of the journals was undertaken to such an extent that for the final machining an oversize of for example 1.8 to 2 mm was still to be removed. In the case of the present invention is the rough machining until in a region of for example 0.5 to 0.7 mm oversize relative to the final dimension carried out. An almost purely fine-grinding operation remains as finish grinding operation, without pre-grinding having to be carried out. In final grinding according to the invention any modifications of the grinding conditions to be adapted are merely implemented by way of the advance and cutting speed of the finish grinding machine.
Thus the advantage of the method according to the present disclosure lies inter alia in the optimization of the production operations so that the significantly smaller size of the flat shoulders of the pin bearings can also be completely achieved by grinding, whereas the significantly higher flat shoulders of the central journals are now pre-turned instead of pre-grinding, turn broaching or trochoidal milling. Thus the effect according to the present disclosure is greatest, without loss of precision and production time or production costs. In this way the centrally disposed high flat shoulders can be produced, preferably simultaneously, by means of cost-effective tools.
The method according to the present disclosure implements a complete machining from forged or cast blanks of a crankshaft, wherein at least the unmachined central pins and crankpin journals as well as the respective flat shoulders thereof which surround the journals, i.e. enclose the journals between them, are completely machined. Therefore complete machining, in particular for the said sections of the crankshaft, is understood to be machining of the rough contour of the blank the crankshaft to the final dimension thereof. According to the present disclosure the flat shoulders associated with the central journals are turned. In this case the turning takes place by means of conventional rotary tools, which are fixed and for purpose of turning are brought into engagement with the rotating crankshaft. Then the central journals as well as the crankpin journals and the flat shoulders thereof are rough-ground. The grinding of the central journals takes place so that in this case the grinding wheel no longer comes into engagement with the flat shoulders present there. As a result the grinding wheel is considerably relieved of load and can rough-grind the oversize on the blank to nearer to the final dimension. Thus after the rough grinding of the journals an oversize remains which is smaller than an oversize necessary for conventional known finish grinding of journals and flat shoulders. With regard to the oversize which is to be removed by the rough grinding according to the present disclosure and the final grinding according to the present disclosure, these two operations are not comparable with the operations used in the prior art. By comparison with the conventional final grinding, the finish grinding according to the present disclosure is relieved of load by the oversize of which more is removed by the rough grinding according to the present disclosure by comparison with the known oversize.
The complete machining is then concluded, as after the rough grinding the central journals and the crankpin journals and preferably also the flat shoulders of the crankpins are finish-ground to the final dimension from the smaller oversize achieved or left by the rough grinding. On the basis of the complete machining, which according to the invention is optimized in relation to the oversize, of at least the journals and flat shoulders of forged or cast blanks of a crankshaft, in addition to a significant reduction in the cycle time in the production of a crankshaft a higher degree of precision thereof is achieved. It is particularly preferable if the flat shoulders of the crankpins are already ground to the final dimension in the rough grinding.
As the flat shoulders, in particular the central journals and in particular all journals, are turned simultaneously, the production time can be further reduced. The loading exerted on the crankshaft during the machining of the crankshaft can be further reduced by the simultaneous turning of all flat shoulders, in particular of the central journals, which has a positive effect on its quality.
The flat shoulders of the central journals are preferably turned in groups. In this case a first group is turned by advancing the rotary tools from a first side of the crankshaft and a second group is turned from a second side of the crankshaft opposite the first side The simultaneous turning of the flat shoulders in groups from two opposing sides of the crankshaft has the advantage that machining forces introduced into the workpiece during turning can be compensated for at least to a certain extent.
Preferably the two groups may each comprise all flat shoulders the central journals. This means that all the flat shoulders can be turned by rotary tools from both sides of the crankshaft, so that an almost perfect compensation for the rotary forces introduced into the workpiece is possible.
In order to save even more cycle time, all central journals are preferably ground simultaneously. For this purpose a number of grinding wheels corresponding to the number of central journals to be ground simultaneously is simultaneously brought into engagement.
The tools for turning preferably have indexable inserts made of hard metal or polycrystalline diamond; the grinding is carried out by means of galvanically coated grinding wheels or ceramic bonded CBN grinding wheels.
In the method according to the present disclosure the rotary tools are preferably brought into engagement as a first set along one side of the crankshaft, and, by turning, the flat shoulders and incisions at the junction between the bearing journal and the respective flat shoulders are turned. The term “preset turning tools” should be understood to mean that, with the turning tools in one single turning operation with radial advance of the turning tools—relative to the axis of rotation of the crankshaft—the required dimension, namely either the final dimension after the pre-turning operation or the finish dimension after the final turning operation, is reached.
For further improvement of the method according to the present disclosure, a second set of turning tools is preferably added to the first set of turning tools along a side of the crankshaft opposite said first set, wherein the second set is brought into engagement simultaneously with the first set and turns the flat shoulders and incisions. Due to the opposing arrangement of a row of turning tools, not only can the mechanical loading of the turning tools be optimized, but also the forces introduced by the turning tools on the component can also be reciprocally compensated, so that the crankshaft is subjected to a minimal bending load during machining. As a result the manufacturing precision on the crankshaft can also be increased. The respective set is disposed on a carrier.
The indexable inserts made from hard metal or polycrystalline diamond which are used for the turning have the advantage that not all turning tools have to be changed when they are worn, but that only the indexable inserts directly performing the turning operation have to be replaced.
According to a further preferred embodiment of the present disclosure invention, in the method according to the present disclosure the end faces, i.e. the flat ends on the flange side and the journal side of the crankshaft, are produced to a finish dimension by means of milling on a milling centre located before the turning centre. Centring holes are also introduced here for later positioning in the turning centre and the grinding machines.
After the rough grinding a hardening and/or rolling of radii and/or drilling of oil channels and/or nitriding are carried out. These additional steps, which of course form part of the complete machining of the crankshaft, are advantageously implemented before the finish machining thereof.
At the end of the turning of the flat shoulders of the central journals, i.e. immediately following the turning of these flat shoulders, an incision is preferably turned with the same turning tools, as it were in a single operation, wherein this undercut produces the junction between the flat shoulders and the surface of the central journals which extends substantially perpendicular to the flat shoulders. These incisions delimit the region of the central journals which constitutes the immediate bearing surface.
According to a further aspect of the present disclosure, a production line is provided by means of which complete machining at least of unmachined central journals and crankpin journals as well as the respective flat shoulders of forged or cast blanks of a crankshaft which surround the journals, i.e. enclose the journals between them, is carried out. In this connection complete machining should be understood to mean at least the complete machining of the unmachined central journals and crankpin journals and the respective flat shoulders thereof, starting from a forged or cast blank of a crankshaft. According to the present disclosure the production line has a turning centre in which the crankshaft to be machined is fixed. The turning centre of the production line has a linear carrier which extends coaxially with respect to the longitudinal axis of the crankshaft and on which a number of turning tools is mounted such that at least a plurality of flat shoulders of main bearing points or central journal regions can be turned, at least pre-turned, simultaneously. Thus the flat shoulders of the main bearing points should be machined as simultaneously as possible and finished as simultaneously as possible. In this case the turning tools are designed so that at least a pre-turning operation can be carried out. In this case “pre-turning operation” is understood to mean that the greatest possible amount of material is removed from the blank, leaving at most an oversize which is very small relative to the finish dimension and which in the course of subsequent finish machining necessitates a significantly reduced chip volume and reduced machining forces which when introduced into the crankshaft lead to reduced bending forces or bending moments during the finish machining.
The production line also at least includes a rough grinding machine which is disposed after the turning centre and of which the rough grinding wheel has a width which is less than the distance between the flat shoulders between which the central journal to be ground extends. This smaller width of the rough grinding wheel means that only the immediate bearing region is ground, so that during grinding a linear contact is produced which can be readily cooled and lubricated, so that on the one hand the grinding forces are moderate and on the other hand a good grinding result relative to the surface to be ground can be achieved. Due to these improved grinding conditions, in which the rough grinding wheel no longer comes into contact with the already finish-turned flat shoulders of the respective journal point, until on a small oversize the rough oversize of the unmachined crankshaft can be removed by the g machine to leave a small oversize. The small oversize is an oversize which is only delivered for fine machining to a final dimension in a finish machining operation. In this way the rough grinding machine considerably relieves the load on the finish grinding process provided downstream.
The advantage of such a production line is that the substantial wear on the grinding wheels which is otherwise present in the rough grinding or pre-grinding of the flat shoulders no longer occurs, because this part of the pre-machining is performed separately on the turning centre by the turning tools. Since the flat shoulders of the central journals or the main bearing points are considerably larger are than those of the pin bearing points, the serious effect on the wear on the grinding wheels is much more greatly reduced by the production line according to the present disclosure than in the case of grinding of the flat shoulders of the pin bearing points. This may be accepted in the interests of overall economy of the complete machining of crankshafts. For example in car crankshafts the flat faces on the main bearing points have a radial height of approximately 12 mm, whereas the flat faces on the pin bearing points only have a height of approximately 5 mm. These figures are of course by way of example and vary from crankshaft to crankshaft, but should merely serve to clarify the relationship between the height of the flat shoulders of the main bearing points and the pin bearing points.
In order to achieve a high production efficiency, the production line preferably has on the carrier on the turning centre a number of turning tools corresponding to the number of flat shoulders of the main bearing points. The turning tools can preferably be preset individually, so that in one single turning operation the desired final dimensions are achieved, i.e. either the final dimensions for the pre-turning or the final dimensions for the finish turning.
On the turning centre a first and a second carrier are preferably provided, which are disposed with their longitudinal axes parallel to one another and coaxially with respect to the longitudinal axis of the crankshaft, wherein the second carrier is located on a side of the crankshaft opposite the first carrier. Both carriers also have a plurality of turning tools, in particular a number of turning tools which corresponds to the number of flat shoulders the main bearing points. These turning tools disposed on the carrier can be simultaneously brought into engagement with the flat shoulders.
The turning tools preferably have a shape by means of which not only the flat shoulders can be turned, but also by means of which the undercut present in addition to the actual bearing region of the central journals in the junction between the flat shoulder and the diameter region can be finish-turned as an incision. This has the advantage that in one single turning operation, as it were in one go, not only the flat shoulders but also this incision can be produced. According to a modification of the present disclosure the turning tools preferably have indexable insert made from hard metal. Even more preferably, the turning tools have indexable inserts which are made from polycrystalline diamond. The advantage of polycrystalline diamond is that the service life is longer, therefore the production process can be optimized in terms of cost, wherein the gain in service life calculated on the basis of the overall costs of the production process is greater than the loss due to the higher costs of the indexable inserts by comparison with indexable inserts made from hard metal.
On the production line a finish grinding machine 18 is preferably disposed downstream of the rough grinding machine 4. On this finish grinding machine 18, which so constructed that a relatively small oversize 23 left by the rough grinding machine 4 can be removed by grinding, the central journals 10 and the crankpin journals 11 can be finish-ground to the final dimension, preferably in a single operation. On this finish grinding machine the flat shoulders of the crankpins 12 can preferably also be finish-bound in just such a way if they have not yet been ground to the final dimension on the rough grinding machine. This would be possible for example if no special quality requirements have to be set for the flat shoulders of the crankpins 12. Then the finish grinding machine 18 can be spared the step of finish grinding of the flat shoulders of the crankpins 12.
Thus the method according to the present disclosure and also the production line according to the invention offer a cost-effective and highly efficient production method which meets the current precision requirements appropriate manufacturing processes for mass production of crankshafts.
In order further to increase the effectiveness in production, the rough grinding machine preferably has a number of rough-grinding wheels which corresponds to the number of central journals to be rough-ground. This is possible in particular with the central journals which constitute the main bearing points, since these central main bearing points should have equal dimensions as far as possible based on the common longitudinal axis of the crankshaft.